Rotary drive mechanisms are used as prime movers for various applications including automobile engines, electric motors of electric cars and industrial motors, and the like. For various kinds of the rotating drive mechanisms, there is a demand for simple and accurate measurement of a physical quantity such as a transmitted torque because such measurement is useful for analyzing drive mechanisms and obtaining a better understanding of the operating condition. Measurement of torque is particularly useful in the case of automobile engines, for example, because by measuring the torque at the engine, the transmission, propeller shaft, differential gear and other points of the drive system, it is possible to control the ignition timing for the engine, the amount of fuel injection, the timing for transmission shift, the gear ratio, and the like. In the case of industrial motors, for example, accurate torque measurement may provide data for optimizing control and diagnosis of rotary drive systems, thereby improving energy efficiency and driving characteristics.
Various kinds of torque detecting apparatus have conventionally been proposed, one of them being an apparatus for noncontactingly measuring the torque transmitted through a rotary magnetic material. However, conventional contactless sensors have lower sensitivity and the sensitivity is substantially dependent on the material properties of the shaft. In other words, use of direct magnetostrictive effect for measuring torque of large shafts requires complex sensor arrangements, difficult sensor calibration procedures and typically results in measurements with limited accuracy.